Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
lOS'~925
In the discontinuous polymerisation of vinyl chloride in aqueous
emulsion it was originally necessary - as with other vinyl chloride
polymerisations - to open the polymerisation kettle between polymerisation
cycles~ in order to clean it. Though, before opening the kettle, the content
of vinyl chloride in the interior of the kettle had been reduced to a minimum,
by evacuation and by flushing with nitrogen, it was unavoidable that on open-
ing comparatively large amounts of nomeric vinyl chloride were emitted into
the atmosphere.
Introduction of automatic hydrodynamic cleaning, as described, for
example, in Austrian Patent Specification 305,724, makes it unnecessary to
open the kettle to clean it. A pressure-resistant hydrodynamically operating
cleaning device is connected to the kettle, for example above the reflux
condenser, and this device is moved into the kettle, if required, between the
polymerisation cycles. Manual cleaning work is in this way avoided.
In spite of the use of this hydrodynamic method of cleaning, the
emission of monomeric vinyl chloride between polymerisation cycles in large
kettles of capacity 60 m or above has hitherto been unavoidable.
In modern processes for the discontinuous polymerisation of vinyl
chloride in aqueous emNlsion high-concentration di~persions, containing at
least 48 per cent by weight of monomer and/or solids, are used, for reasons
of saving energy on spray drying, and better utilisation of the production
equipment.
At the same time, it is desirable to operate using as little `
emulsifier as possible~ since high emulsifier contents have a very adverse
effect on the end products, and on processing.
Modern processes therefore use monomer and/or solids contents of
at least 48 per cent by weight with emulsifier contents of 0.9 to 0.4 per
cent by weight, relative to monomer. Such a process is described in West
German Auslegeschr~ft 1~964~029.
However~ on withdrawing such highly concentrated dispersions of
low emulsifier content from large kettles of at least 60 m3 capacity, problems
arise.
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Attempts to proceed analogously to the conventional method of
working in small kettles, that is to say to p~mp out the dispersion, fail in
the case of large kettles.
If a displacement pump which does not subject the dispersion to
severe mechanical conditions is used, at most 20 to 30 tonnes of dispersion
can be conveyed per hour. In that case it would be necessary to expend, ~ere-
ly for emptying the kettle, about 3 hours with, for example, a capacity of
100 m3, or even 6 hours with the more usual 200 m3 kettles. The expenditure of
this amount of time merely for pumping out the dispersion would disproportion-
ately increase the down times between polymerisation cycles and would sub-
stantially reduce the space-time yield from the large kettle.
However, it is not possible to use high speed centrifugal pumps,
since highly concentrated dispersions of low emulsifier content do not with-
stand the associated high mechanical stresses.
Hence, large kettles have been emptied by releasing into the gas-
ometer, after the end of a polymerisation cycle, the vinyl chloride monomer ~ -
still present under pressure, applying a vacuum and finally forcing the highly
concentrated dispersion through the discharge device by means of nitrogen.
The vinyl chloride/nitrogen mi~ture remaining in the kettle after it had been
discharged than had to be blown off into the atmosphere since separation of
the mixture with a view to reusing the vinyl chloride is too expensive.
In this way, considerable amounts of monomeric vinyl chloride were
emitted. This method therefore implied both economic losses and pollution of -
- the en~ironment. However, the procedure described also required the expendi-
ture of much time in the case of large kettles, since the process of evacuation,
including the requisite vacuum tests in the large kettle, requires several
hours.
Tbis invention seeks to overcome the disadvantage of the earlier
procedure, by providing a process for the manufacture of emulsion polyvinyl
chloride, which avoids emission of vinyl chloride, by discontinuous polymeri-
sation in a highly concentrated aqueous emulsion of low emulsifier content in
the presence of water-soluble free_radical catalysts, in pressure kettles of
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at least 60 m3 capacity, which is characterised in that after completion ofthe polymerisation the polymer dispersion is conveyed, under a vinyl chloride
pressure of 0.1 to 2.5 atmospheres, from the polymerisation kettle into the
discharge device and the polymerisation kettle is left unopened between poly-
merisation cycles and, if necessary, is automatically cleaned hydrodynamically,
in a manner which is in itself known, between the polymerisation cycles. Pre-
ferably, the process is carried out in pressure kettles of at least 100 m3
capacity.
In a further preferred embodiment of the invention, the process
followed is such as to avoid caking to the walls. In that case, the effort
required for cleaning is also reduced and at times completely avoided.
In detail, the procedure according to the invention is carried out
as follows.
When, at the end of the polymerisation cycle, the pressure in the
polymerisation kettle has fallen to 4 - 5 atmospheres gauge, the pressure is ~ -
equalised between the polymerisation kettle and the discharge vessel via a
pressure-equalising line~ so that the pressure in both containers is 2.5 to
3.5 atmospheres gauge. After the pressure has been equalised, the shut-off
device in the pressure-equalising line is closed. Now, the shut-off device
between the polymerisation kettle and the discharge vessel, on the one hand~
and the shut-off device between the discharge vessel and the gasometer on the
other~ are opened, and the dispersion passes, under a pressure difference of -
0.1 to 2.5 atmospheres, preferably 1 to 2 atmospheres, into the discharge
vessel. The release of pressure from the discharge vessel must proceed at
such a rate that, bearing in mind the changing levels in the polymerisation
vessel and the discharge vessel, a pressure difference of 0.1 to 2.5 atmo-
spheres, preferably 1 to 2 atmospheres, remains preserved.
If, towards the end of the discharge of the dispersion~ the pres~-
u~e in the polymerisation kettle should no longer suffice to convey the dis-
persion into the discharge vessel, the applied pressure can be increased byintroducing a small amount of liquid vinyl chloride into the polymerisation
kettle. When the polymerisation kettle is empty, this fact is indicated by a
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lOS'~925
level indicator in the discharge line. Pressure equalisation between the
polymerisation kettle and the discharge vessel is necessary because, at a
pressure difference of more than 2.5 atmospheres, the dispersion of high solids
content and low emulsifier content tends to precipitate. Pressure kettles for
carrying out the polymerisation have a capacity of at least 60 m3. Those of
capacity 100 to 250 m3, especially 100 to 200 m3, are particularly suitable.
The pressure kettles (polymerisation kettles) in general have the industrially
customary shape of standard kettles. Because of the size of the reactor, the
stirring device is driven from the bottom. Appropriately, the polymerisation
kettle is provided with a reflux condenser, since at the sizes of kettle in
question cooling through the kettle wall in general does not suffice to remove
the heat of polymerisation, if the polymerisation is to be carried out at a
sufficiently high rate.
me device for hydrodynamic cleaning is suitably located above the
reflux condenser and the cleaning instrument is, if required, introduced
through a channel in the zone of the reflux condenser (compare Austrian Patent
Specification 305,724, Claim 5). The cleaning device is of pressure-resistant
design. The discharge vessel must have a volume at least equal to that of the
`~ polymerisation vessel. Suitably, the volume of the discharge vessel is 1 1/2 i#
j 20 times that of the polymerisation kettle.
The figure shows a polymerisation unit suitable for aarrying out
` the process according to the invention. The polymerisation kettle (1) is con-
nected to the discharge vessel (2) by the discharge line (6) and the pressure-
equalising line (3). Both lines are provided with appropriate shut-off de-
vices (4) and (5~. The discharge line (6) possesses a measuring instrument
(10). This causes the shut-off device (5) to close when the dispersion has
been emptied out of the polymerisation kettle (1). A gasometer (9) is con-
`; nected by a line (7) to the discharge vessel (2) via the shut-off device (8)
`~ and to the polymerisation kettle (1) via the shut-off device (14). Further-
mor~e, the polymerisation kettle (1) possesses a manhole (11) and a vacuum
line (13) with a shut-off device (12) and a nitrogen line (16) with a shut-
off device (15). The pressure in the discharge vessel (2) is regulated via
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the shut-off device (8), by means of a pressure-measuring device (17).
The polymerisation is directed so that dispersions of at least 48
per cent by weight solids content and at most 0.9 per cent by weight, in
general 0.7 per cent by weight, of emulsifier content are prod7lced. A pro-
cedure which permits the manufacture of such dispersions of high concentration
and low emulsifier content is descri~ed in West German Auslegeschrlft 1~964~029
according to which the emulsifier is added continuously, in a specified way,
during the polymerisation cycle.
Catalysts which can be used are the water-soluble compound
customary in emulsion polymerisation, such as hydrogen peroxide and sodium
persulphate, in the customary concentration. Preferably, potassium persulphate
is employed. The concentration is 0.1 to 5.0 per cent by weight~ preferably
0.5 to 2.0 per cent by weight, based on the monomer. The known types of
emulsifier can be employed. In particular, it is possible to use ionic
emulsifiers, for example salts of carboxylic acids, such as sodium caprate,
i sodium laurate~ sodium myristate and sodium palmitate. Salts of primary and
~ secondary alkyl-sulphates, for example sodium capryl.sulphate, sodium lauryl-
. .
sulphate~ sodium myristyl-sulphate and sodium oleyl-sulphate are also suitable.
- It is al90 possible to use sulphates of esterified polyoxy-compounds such as
i 20 mono-fatty acid esters of glycerol-sulphuric acid esters, salts of primary
`~ and secondary alkylsulphonates such as sodium ethylsulphonate, sodium
stearylsulphonate and sodium oleylsulphonate, n-alkanesulphonates with a
statistical distribution of the sulphonic acid group and chain length of
C13-C17, and the like. It is also possible to emply alkylarylsulphonates, for
example the Na salt of P-n-dodecylbenzenesulphonic acid.
Furthermore~ mixtures of emulsifiers can also be used. Auxiliaries
- can also be admixed additionally to the emulsifiers mentioned, for exaæple an
` alcohol such as lauryl alcohol, and esters such as sorbitol monolaurate and
carboxylic acid esters of glycol.
The polymerisation temperature is 40 to 70C, depending on the
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desired ~-value.
Caking-on of polymer can be avoided practically completely by
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measures such as those described in West German Patent Application P 24 05
- 978.7, that is say by using polymerisation kettles wherein the surfaces of
inner walls and of fittings and coolers present in the kettle consist of
nickel. If such measures are used, the effort required for cleaning is also
. reduced. ~-
.. i The polymerisation batch can be heated by direct introduction of
i steam, as described in West German Patent Application 2,257,025.
The examples which follow serve to explain in more detail the : -
:. procedure according to the invention and the advantages achievable thereby.
E1~amDle 1 , '-
A part of the water required, which contains 15 kg of potassium ~.
persulphate (0.25 per cent by weight, based on monomer) is introduced into
a 200 m3 polmerisation kettle (1), which is under slight excess pressure of
i vinyl chloride. After heating up to the polymerisation temperature of 54C
.~ by direct introduction of steam, 65 tonnes of vinyl chloride as well as water
` required for emulsification, and hydrogen peroxide (as required) are pumped
continuously, over the course of 4 hours, into the polymerisation kettle (1).
. After completion of the pol~merisation (when the pressure drops), at 4.5
atmospheres gauge, the pressure between the polymerisation kettle (l? and the
discharge vessel (2) is equalised via a pressure-equalising line (3), so that
a pressure of about 3 atmospheres gauge prevails in both vessels. The gas
(pressure) equalising line (3) between the polymerisation kettle and the dis- .-.
charge vessel (2) is then closed by actuating the shut-off device (4). The ~ -
shut-off device((5) in the line (6) between the polymerisation kettle (1) and
the discharge vessel (2), and the shut-off device (8) in the pressure release
line (7) between the discharge vessel (2) and the gasometer (9) are now opened
and the dispersion flows into the discharge vessel (2). When the polymerisation
kettle (1) is empty, as is indicated by a radioactive level indicator (10) in
the discharge line (6), the flushing and cleaning process by means of the
.automatic hydrodynamic cleaning instrument commences in an atmosphere of vinyl
chloride. The water used for.cle.aning i~ ~reed..from.vinyl chloride in a
separate column and is subsequently collected and reused. After the cle~:ning
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process, the polymerisation vessel is free for the initial introduction of
water for the next batch.
ample 2 (Comparative example)
A 200 m polymerisation kettle (1) which is filled with air is
charged with the water to be introduced initially, and a catalyst dissolved
in water. The manhole (11) is closed, a pressure test is carried out (about
1 hour) and the polymerisation kettle (1) is evacuated by opening the shut-
off device (12) in the vacuum line (13) (about 1/2 hour). The polymerisation
kettle is then brought to the desired temperature, as in Example 1~ by direct
introduction of steam, and vinyl chloride, water required for emulsification
and catalyst solution are metered in over a period of 4 hours. After the
polymerisation has ended (when the pressure drops), the polymerisation kettle
is let down to about 0.5 atmosphere gauge to the gasometer (9) via the line
(7) by opening the shut-off device (14) (about 2 hours)~ and the dispersion
is forced into the discharge vessel (2) by forcing in nitrogen from line(l6)
via the shut-off device (12). Thereafter the polymerisation kettle (1) is
flushed with nitrogen and the manhole (11) is opened for the purpose of
flushing and cleaning with a cleaning instrument which employs water under
pressure. ~;
The loss of vinyl chloride, or the emission of vinyl chloride
into the atmgsphere, is at least 300 kg~ in contrast to Example 1. The
additional time required, as compared to Example 1~ is about 3 1/2 hours.
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